Mast cells were examined in order to evaluate the biologic characteristics, function, and differentiation of mast cell precursors; to evaluate the role of adhesion receptors in mast cell biology; and to understand mast cell cytokines. Mast Cell Precursor Biology Mast cell precursors from murine bone marrow have an obligatory requirement for IL-3 early in maturation, after which they mature into the mucosal phenotype in IL-3 alone, or toward the connective tissue phenotype in c-kit lignad (stem cell factor). Fc(e)RI+ mast cell precursors are also positive for Fc(g)RII/III. Acquisition of Fc(e)RI+ is associated with a diminished proliferative capacity suggesting it is a marker of terminal differentiation. Human CD34+ bone marrow derived cells give rise to mast cells on c-kit- negative fibroblasts only when these fibroblasts have been transfected to produce membrane bound c-kit ligand with or without producing the soluble form of c-kit ligand. Mast Cell Adhesion Receptors Murine mast cell spontaneously adhere to vitronectin through an alpha(v) beta(x) integrin in a calcium dependent process. When mast cells adhere to vitronectin in the presence of IL-3, therir proliferative rate increases. This is the first evidence of an adhesion receptor mediated alteration in mast cell behavior. Murine mast cells express ICAM-1 and VCAM-1 and display heterotypic aggregation with murine lymphocytes. Mast Cell Cytokines IL-6 is produced in mouse skin following IgE-mediated degranulation demonstrating that IL-6 of one of the mediators of inflammation that follows allergic reactions.